Lung volume measurement is usually done as part of pulmonary function testing, which is often needed for people with lung disorders such as asthma, COPD, and emphysema. Certain lung volumes can be measured during regular spirometry testing, but calculating residual lung volume requires special techniques.[1] Residual lung volume represents the amount of air left in your lungs after forced exhalation (breathing out as much as you can). Residual lung volume is not actually measured directly, but it can be calculated using special methodologies. Restrictive lung diseases, such as pulmonary fibrosis, asbestosis and myasthenia gravis are characterized by reduced residual lung volumes.

Steps

Part 1

Understanding Lung Volumes

1

Realize that residual lung volume is not your tidal volume. Respiratory rate is how many breaths you take in a minute. At birth, the average human respiratory rate ranges from 30 – 60 breaths per minute, whereas its much lower at 12 – 20 breaths per minute in adults.[2] Tidal volume is the amount of air inhaled or exhaled during normal respiration (breathing), which amounts to about 0.5 L in both men and women.

Tidal volumes increase during deep sleep and with relaxation, but decrease with stress, nervousness and panic attacks.

In contrast, residual lung volume does not fluctuate with states of consciousness or mood.

Men have slightly higher residual lung volumes because they tend to have larger bodies and lungs.

2

Know that residual lung volume is not the same as functional residual capacity. When you exhale while breathing normally, the volume of air left in your lungs is called the functional residual capacity, which is NOT your residual volume.[3] Instead, the residual volume is the air left in your lungs after a forced exhalation, which indirectly measures the strength of your respiratory muscles (diaphragm, intercostal muscles, etc.) as well as the health of your lung tissues.

Shallow breathing (due to asthma, for example) results in a larger functional residual capacity, whereas a larger residual lung volume is a sign of good fitness and healthy lung tissue.

Average functional residual capacity is about 2.3 L of air in men and 1.8 L in women.

In contrast, residual lung volume is always lower than functional residual capacity — 1.2 L for men and 1.1 L for women.

3

Remember that residual lung volume is not easy to measure. Although residual lung volume is the amount of air left in your lungs after you completely breathe out, the reality is that it's practically impossible to do so on your own. As such, residual lung volume isn't measured like tidal volume is, for example; instead its calculation has to be done via indirect methods such as closed circuit dilution (including helium dilution), nitrogen washout and body plethysmography.[4]

In absence of special testing, residual lung volume can be estimated based on a proportion of body mass or vital capacity, as well as a person's height, weight and age; however, these estimates are not particularly accurate and not as helpful for determining lung diseases.

Residual lung volume is reduced with restrictive lung disease, but it also changes somewhat in response to pregnancy, significant weight gain and muscle weakness due to aging.

Part 2

Calculating Residual Lung Volume

1

Get a referral to a medical specialist who can conduct a helium dilution test. If your family doctor thinks you have a restrictive lung disease, they will refer you to a respiratory (lung) specialist, also known as a pulmonologist, for further testing. The pulmonologist may conduct a helium dilution test. This inert gas dilution method uses helium to directly identify your residual lung volume. To start the test, you'll exhale normally and then be connected to a closed system containing known volumes of helium and oxygen.[5] Once connected, you breathe in the helium and the amount exhaled is measured. The difference between the two volumes of helium is a pretty accurate estimate of your residual lung volume.

Helium is an inert, colorless, odorless, tasteless gas and is non-toxic to your lungs, so there are no health concerns related to this exam.

This technique may underestimate residual lung volume because it measures only the lung volume that communicates with the airways. This can be problematic for patients with severe airflow limitation.

2

Consider the nitrogen washout technique. You will also need a referral to a pulmonologist to have this test done, which measures the air remaining in your conducting airways. To start the test, you'll exhale normally and then be connected to a spirometer that contains 100% oxygen. You'll then breath in deeply and exhale as strongly as you can, and the spirometer will measure the amount of exhaled nitrogen compared to the entire volume of exhaled air.[6] The halfway point of the percentage of exhaled nitrogen allows the doctor to figure out the amount of gas you expelled, which is equal to the residual lung volume.

Recall that the air we normally breathe in is about 21% oxygen and 78% nitrogen.[7] This test forces you to breathe 100% oxygen and then measures the amount of nitrogen exhaled, a predetermined percentage of which represents residual lung volume.

Like the helium dilution technique, nitrogen washout can also underestimate residual lung volume in patients with severely restricted airflow.

3

Have a body plethysmography done for best accuracy. This very accurate method for measuring residual lung volume uses a plethysmograph, which is an enclosed instrument (a small chamber you sit in) used for recording an organ’s volume change. Once inside an airtight plethysmograph — it sort of looks like a small phone booth — you'll be asked to normally exhale, then inhale against a closed mouthpiece. As your chest wall expands, the pressure within the plethysmograph rises, which is calculated.[8] Then you'll exhale as hard as you can out through the mouthpiece. The difference in pressures represent your residual lung volume.

Body plethysmography uses Boyle's gas law (pressure and volume of a gas have an inverse relationship when temperature is constant) to determine residual lung volume and other lung volumes.

Body plethysmography is considered more accurate than gas dilution methods for calculating lung volumes, particularly if the lung is obstructed.[9]